Recently, I have been working on a project using Django as an API server, and there is one special API which generates a file with arbitraty generating time and size. When calling this API, I always got a 502 error. In the beginning, I thought it might be the problem of timeout setting, so I changed the timeout settings of both Django and Apache (using as reverse proxy) to 300 seconds. However, I still received 502 error.
Then I started to investgate the cause and realized the meaning of
worker setting of Gunicorn. After I altered the worker type from
sync to asynchronous worker, this
particular API call acts normally not only its responses time but also
its execution speed.
In order to let both myself and others to realize the meaning of Gunicorn workers, I wrote this post as a reminder.
Types of Gunicorn workers
Gunicorn uses pre-fork worker model, which means it uses a central
master process to manage a set of worker processes. By default,
Gunicorn uses synchronous (
sync) worker, but sync worker does not
support persistance connections, which means you shouldn’t do
anything that takes vary amount of time just like this case. 
Suggest in Gunicorn , you can do one of the following solutions:
- a buffering proxy
- asynchronous workers
For ease I chose asynchronous workers, and this solved the problem.
If your application will make a long blocking calls, e.g., external
web services, you can consider switch to